Alone in his Marylebone flat one September night in 1971, the amateur radio enthusiast Robert Rowlands found himself listening in on one of the most audacious British bank jobs of all time.

Turning his radio dial, he picked up a two-way conversation between a man named Bob, acting as a lookout, and another man complaining about fumes from cutting equipment. Amazed, he called the police to tell them what was going on.

Today, as The Bank Job, a Hollywood film based on the famed Baker Street raid, is about to be released, Rowlands has talked of the heist for the first time in more than three decades to defend the honour of the late Princess Margaret from the screenplay's insinuations. 'I was the person who brought the robbery to the notice of the police as it was happening, and I still have the recordings I made at the time,' he said.

The incident made Rowlands the highest paid news interviewee of the era, as the media scrambled to find out about the caper that netted £500,000, the equivalent of £5m today and the biggest haul ever at that time. Details of the Lloyds Bank raid, known as the 'walkie-talkie robbery', were cloaked in secrecy, because the police imposed unusual reporting restrictions. Rowlands believes the film-makers behind The Bank Job have exploited this mystery to distort the truth and insult the memory of the Queen's sister by suggesting that the bank vault contained compromising photographs of the princess. 'The film is an amusing series of misconceptions, dragging in royalty. I am in touch with the princess's solicitors,' said Rowlands, played in the film by Angus Wright.

The script for The Bank Job, which stars Saffron Burrows and Jason Statham, was written by British writers Ian La Frenais and Dick Clement, best known for their TV series The Likely Lads, Porridge and Lovejoy

Last spring the writers told The Observer they had been drawn to the story by transcripts of the conversations between the bank raiders that Rowlands recorded and which were reprinted in newspapers. They had not managed to track the radio ham down.

'The gang had walkie-talkies and look-outs on the roof,' Clement said then. 'I read about the robbery at the time and the great remark that Ian and I remember was one of the lookouts saying: "I'm off home now, I'm cold and hungry." A gang member said: "You can't go now, we're almost there." And the reply was: "Money may be your god, but it's not mine and I'm fucking off".'

The film tells how the gang took over an empty shop nearby and tunnelled 40ft under a restaurant called the Chicken Inn before breaking through the bank's floor and entering the safety deposit vault. While Rowlands admits The Bank Job has been written in a 'tongue in cheek' style, he believes it has failed to get to the core of the story. 'The film is probably entertaining,' he said, 'but nothing to do with the true events.'

After overhearing the gang, Rowlands called Marylebone police. An officer told him to tape the conversation. The film explains this by alleging the crime had been set up by MI5 to get the photographs and free the royal family from the threat of blackmail. While La Frenais and Clement do not give details of the nature of the photographs, an unnamed princess is said to be depicted frolicking on an island hideaway.

In 1973 four men were jailed for the raid. But Rowlands believes that even today no one has risen to the challenge left by the gang in the Baker Street bank, not far from the fictional home of the country's most famous detective. Scrawled inside the safe were the words: 'Let Sherlock Holmes try to solve this.'